1. Field of the Invention
The present invention relates to a magnetoresistive film used for a magnetic sensor, which detects a magnetic signal from a magnetic recording medium and converts it to electrical information, especially for a read-only magnetic head of a hard disk unit, magnetoresistive sensor element, and the like.
2. Description of the Related Art
Hitherto, a magnetic head has been used as a means for converting an electric signal to a magnetic signal, recording it as information on a magnetic recording medium, and reproducing the magnetic information in the form of the electric signal. To record information, such a magnetic head generates a magnetic field by exciting a ferromagnetic material, which has high permeability, by energizing a coil so as to magnetize a magnetic recording medium for recording. To reproduce the information, the magnetic head detects, as a signal, a voltage generated in a coil by a magnetic flux flowing through the ferromagnetic material due to the magnetic field generated from the magnetic recording medium. This type of magnetic head is called an inductive head which is typically represented by a metal-in-gap ferrite head and a thin-film head which uses a magnetic thin film.
The reproduction sensitivity of these inductive heads decreases with decreasing magnitude of the magnetic field generated from a magnetic recording medium. Hence, it is considered that the inductive heads will be no longer good in a case where the recording density increases and the recording bit, which is the unit of magnetic information formed on a magnetic recording medium, decreases with a resultant reduced leakage magnetic field generated by the bit. To cope with this difficulty, a magnetoresistive head with higher reproduction sensitivity has recently become used for the reproducing section of a magnetic head.
For the magnetoresistive head, a Ni--Fe type Permalloy ferromagnetic film, the electrical resistance of which changes as magnetic field changes, is used; it is expected to provide reproduction sensitivity which is at least three times that of the inductive heads. This head, however, makes use of the characteristic of the Permalloy film in which the electrical resistance changes in dependence on the magnetization direction of the Permalloy film; therefore, magnetoresistance ratio, which is the index of reproduction sensitivity, is only 2% at the most. Recently, there has been proposed a spin valve multi-layer film which has multiple layers as shown in FIG. 5 and which makes use of the scattering occurring at the interfaces defined between the layers (Japanese Patent Laid-Open No. 4-358310). The spin valve multi-layer film provides the magnetoresistance ratio which is several times higher than that of the conventional Permalloy element.
The aforesaid multi-layer film is formed of a second ferromagnetic layer, the magnetization direction of which is fixed by an antiferromagnetic layer, and a first ferromagnetic layer which is separated from the second ferromagnetic layer by a non-magnetic layer. The magnetization direction of the second ferromagnetic layer is fixed by a unidirectional anisotropy field caused by the exchange coupling between the antiferromagnetic layer and the second ferromagnetic layer; weak ferromagnetic coupling occurs between the second ferromagnetic layer and the first ferromagnetic layer having the unfixed magnetization direction, both of which first and second ferromagnetic layers are separated from each other by the non-magnetic layer. The ferromagnetic layer with the unfixed magnetization direction is magnetically turned to the direction of the applied magnetic field by applying small magnetic field (H.sub.1), whereas larger applied magnetic field (H.sub.2) is required to magnetically turn the second ferromagnetic layer with the fixed magnetization direction. The electrical resistance is smaller when two ferromagnetic layers are in the same magnetization direction; the electrical resistance increases in a magnetic field which is larger than H.sub.1 but smaller than H.sub.2 and when the magnetization directions of the two layers are antiparallel.
In a case where the magnetoresistive element is used as a magnetic sensor, the magnetic sensor acts to detect a magnetic field which is less than H.sub.2 and the polarity of which switches. For making this possible, the ferromagnetic layer with the unfixed magnetization direction is required to exhibit good soft magnetic characteristic typically represented by a small coercive force, and the ferromagnetic layer with the fixed magnetization direction is required to exhibit a large unidirectional anisotropy field brought about by the antiferromagnetic layer and not to be sensitive to a leakage magnetic field from a recording bit.
On the other hand, the multi-layer structure is known to generally exhibit higher electrical resistance than that in a single-layer (J. Phys. F, Met.Phys., 15(1985) No. 2477). In a practically used magnetic sensor element such as a magnetic head, a certain level of extremely small electric current is made to flow therethrough, whereas in the films constituting the element, an electric current having a high current density of approximately 10.sup.7 A/cm.sup.2 is made to constantly flow therethrough. Hence, the high electrical resistance of the multi-layer structure causes the element to generate more heat, leading to deterioration in performance including more electrical noises due to thermal disturbance.
Hence, there has been a demand for a magnetoresistive element having both a lower electrical resistance of the film and a greater magnetoresistance effect (that is, a greater magnetoresistance ratio which is the performance indicator for the element) even in a case where the magnetoresistive element is formed of materials similar to those of conventional elements.